To create a fast- or slow-motion effect, change clip speed. Changing clip speed changes the clip’s duration. Speeding up a clip removes frames, thus shortening the clip duration. In the same way, slowing down a clip repeats frames and thus adds to the length of a clip. For audio clips, a change in speed also changes pitch. The Time Stretch command includes an option to maintain the original pitch of an audio clip at any speed.
You can change a clip’s speed using the Expert view timeline only. Not the Quick view timeline.